Near-infrared electroluminescence beyond 940 nm in Pt(N∧C∧N)X complexes: influencing aggregation with the ancillary ligand X

We present a study of aggregate excited states formed by complexes of the type Pt(N<^>C<^>N)X, where N<^>C<^>N represents a tridentate cyclometallating ligand, and X = SCN or I. These materials display near-infrared (NIR) photoluminescence in film and electroluminescence in NIR OLEDs with lambda maxEL = 720-944 nm. We demonstrate that the use of X = SCN or I modulates aggregate formation compared to the parent complexes where X = Cl. While the identity of the monodentate ligand affects the energy of Pt-Pt excimers in solution in only a subtle way, it strongly influences aggregation in film. Detailed calculations on aggregates of different sizes support the experimental conclusions from steady-state and time-resolved luminescence studies at variable temperatures. The use of X = I appears to limit aggregation to the formation of dimers, while X = SCN promotes the formation of larger aggregates, such as tetramers and pentamers, leading in turn to NIR photo- and electroluminescence > 850 nm. A possible explanation for the contrasting influence of the monodentate ligands is the lesser steric hindrance associated with the SCN group compared to the bulkier I ligand. By exploiting the propensity of the SCN complexes to form extended aggregates, we have prepared an NIR-emitting OLED that shows very long wavelength electroluminescence, with lambda maxEL = 944 nm and a maximum EQE = 0.3 +/- 0.1%. Such data appear to be unprecedented for a device relying on a Pt(ii) complex aggregate as the emitter.

Automated summary

We conducted a study on aggregate excited states formed by complexes of the type Pt(NC)X, where N<^>C<^>N represents a tridentate cyclometallating ligand, and X = SCN or I. We found that the identity of the monodentate ligand strongly influenced aggregation in film. The use of X = SCN promoted the formation of larger aggregates, while X = I limited aggregation to dimers. By exploiting the propensity of SCN complexes to form extended aggregates, we achieved an unprecedented NIR-emitting OLED.